The use of ink-jet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today's inkjet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality, while further lowering cost to the consumer.
An ink-jet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a “printhead” onto a printing medium. Inks normally used in ink-jet recording are commonly composed of water-soluble organic solvents (humectants, etc.), surfactants, and colorants in a predominantly aqueous fluid. When a recording is made on “plain paper”, the deposited colorants retain some mobility, which can be manifest in poor bleed, edge acuity, feathering, and inferior optical density/chroma (due to penetration of the paper). These features adversely impact text and image quality.
The evaporation rate of water-based inks is low and they dry primarily by the adsorption into paper. The adsorption time of an ink is dependent on many factors, such as the ink drop size, paper mesh size, ink viscosity, and the ink spreading coefficient over the paper (that is, the surface tension of the paper minus the surface tension of inks and the interfacial tension of the paper-ink interface). There is a considerable interest in reducing the ink drying time, in particular, for high throughput printers. However, shortening the drying time is normally attained at the expense of the image quality. Thus, adding surfactants to inks can significantly reduce the penetration time. This however decreases the optical density/chroma and the edge acuity of the image, as well as ultimately leading to strikethrough problems, because of the penetration of the colorants in normal and lateral directions of the paper. It would be desirable therefore to separate the colorant and the ink vehicle on paper so that the dyes stay at the surface, while the vehicle is quickly adsorbed. Various approaches have been used to achieve this. Differential dye solubility, that is, using dyes that are soluble in ink but insoluble on paper, has been used with limited success. These approaches take advantage of dye solubility and other chemistry changes, caused especially by pH changes in the ink as it goes into the paper. Dye solubility also changes as water, or other solvent, evaporates from the media. Another drawback of these approaches is that they require the dye to be close to its solubility limit in the ink. When the ink is close to its solubility limit, this invariably creates reliability problems primarily due to decap and kogation. As lower and lower ink drop weights are required in ink-jet printers, these reliability issues become more difficult to solve with the conventional approaches.
To address these problems, methods have been described in which a “fixer” solution, with components to reduce colorant mobility, is deposited on the paper prior to depositing the ink. As described, this “under-printing” of imaging ink uses compositions containing acids, salts, and organic counter ions and polyelectrolytes. Underprinting is defined as applying a transparent liquid on paper just before applying inks. The idea was to apply “a separate reactive component” before the inks in order to improve waterfastness. The “reactive component” reacts with a reactant present in the inks, producing a polymer that binds the colorant and makes it waterfast. Bifunctional acylchlorides and bifunctional amines are given as possible reactive pairs. However, acylchlorides are very reactive and require a nonaqueous ink vehicle. Another example of a reactive pair includes carboxymethylcellulose in the presence of aluminum salt which together form a gel. This reaction is more benign than the first, but has the drawback that carboxymethylcellulose solutions have a high viscosity even without the polyvalent ions present. This makes it difficult to use in an inkjet.
In other disclosures, the underprinting idea has been further developed. Using a cationic “liquid composition”, such as a polyallylamine for underprinting of anionic dyes has been suggested, to achieve a fixer, which reacts broadly with dye based systems as a whole. This fixer moderately improves waterfastness, edge acuity and bleed but fails to achieve chroma and/or optical density improvement as is achieved with the present invention.
A broad class of underprinting liquids for pigments has also been disclosed, including polymer latexes, silica, alumina and titanium oxide particles, polymer resins, buffer solutions, and inorganic salts. All these underprinting liquids destabilize the pigment dispersions by various mechanisms. As a result, the pigment substantially precipitates at the surface of the paper, while the vehicle is quickly adsorbed.
Another system has been described that uses water-soluble dyes and an underprinting fixer. The fixer contains ligand-complexed metal ions (metal ion with an associated ligand). The ink used in this system contains an anionic component, which may be the dye itself. The ink may also contain a polymeric viscosity modifier in the ink. The specific polymeric viscosity modifier disclosed is an ethylene oxide adduct of acetylene glycol—a neutral polymer which unlike the present invention does not interact strongly with the fixer.
Another system disclosed uses dispersed pigment colorants and requires a glycol ether cosolvent in the ink. This is unlike the system of the present invention, which uses water soluble dye colorants with little cosolvent restriction.
Yet another disclosed system uses a dispersed pigment (a pigment and a water-soluble resin for dispersing the pigment). It does not teach that such a water soluble polymer might be used as a “binder” in a water soluble dye based system and in fact refers to binder-like polymers as being undesirable because of increased viscosity, poor discharge stability, and clogging.